Film cameras often use flash devices which output a very bright light for a very short period of time. A strobe light implemented using a xenon gas discharge tube is an example of a type of flash device that might be used with a conventional film camera. In the case of film cameras, the entire film area is normally exposed at the same time and thus a relatively short burst of light is well suited for lighting an image area being captured when film is used as the light sensing material.
Advances in electrical sensors such as CCD (charge-coupled devices) devices capable of sensing received light and converting light captured over a period of time into an electrical value have resulted in digital cameras replacing film cameras in a wide range of applications. Currently, digital cameras are being used in many still and video applications where film cameras were previously used.
While high resolution sensors are becoming common place in digital cameras, sensors used in digital cameras suffer some drawbacks which are not encountered in the case of film cameras. One particular disadvantage is that it is usually not possible to read out the output of all the pixel elements of a sensor at the same time. To address this problem many digital cameras use what is known as a rolling shutter.
Rolling shutters may be implemented as physical shutters which expose different portions of a piece of film or sensor sequentially or electrically. Physical rolling shutters block light from reaching different portions of the sensor at different times with portions of the sensor being exposed in a sequential or “rolling” manner often with a sensor portion corresponding to the top of an image being exposed first with the exposure then moving toward the bottom portion of the image.
In the case of use of an electronic rolling shutter the requirement for a mechanical shutter is avoided and the rolling shutter is implemented through electronic control of the sensor being used. Because electronic rolling shutters avoid the need for moving mechanical parts associated with mechanical shutters, they are often preferred in digital cameras for cost and reliability reasons.
In the case of an electronic rolling shutter, different portions of a sensor are read out sequentially. Prior to the point in time that the data is to be read out from an individual portion of a sensor, the sensor elements which provide individual pixel values, associated with the portion of the sensor that are to be read out are normally reset and electrical charge due to light exposure is then accumulated for the desired exposure time. The reset normally occurs at a point in time which precedes the read out time by an amount of time equal to the desired exposure. In this way, the duration of exposure is controlled despite the fact there is no physical shutter blocking light from reaching the sensor. At the end of the exposure time, the pixel elements of the sensor which are to be read out are read. The reading of the next portion of the sensor will then proceed in a similar fashion until all the pixel elements of the sensor have been read out providing a complete set of pixel values corresponding to an area whose image is being captured by said sensor.
While a single image is generated from the readout of the pixel elements of a sensor, it should be appreciated that when a rolling shutter is used different portions of the image, e.g., frame, will actually have been captured and correspond to different points in time. It should also be appreciated that in the case where a rolling shutter is used the amount of time used to capture the image will in many cases exceed the exposure time.
Since light is being captured by different portions of a sensor over a period of time which is longer than the exposure time in the case of digital cameras using a rolling shutter, strobe type flashes are normally not used since the duration of such a flash is often less than the time period required to capture a complete image.
In digital cameras, LED (Light Emitting Diode) or other light sources which can be kept in continuous operation for an extended period of time are often used to illuminate an area as an image of the area is captured. The ability to keep a LED light source active for extended periods of time allows for illumination of an area during the full time period in which an image of the area is captured. However, such continuous illumination of an area can be energy intensive given that the LED is normally powered for the full period of time in which the image is being captured even though only a small portion of the sensor is being used for light capture during any given time period.
In view of the above discussion, it should be appreciated that there is a need for improved illumination methods and apparatus. It is desirable but not necessary that some methods and/or apparatus be well suited for use with digital cameras. IT would be desirable if some but not necessarily all embodiments support energy efficient illumination, e.g., which are more efficient than conventional approaches which fully illuminate an object area for the full time period in which image capture by a sensor is occurring.